Light emitting diodes (LEDs) are widely used in consumer and commercial applications. Continued developments in LED technology has resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the long service life of solid state devices, have enabled a variety of new display applications, and have even resulted in the use of LEDs in general illumination applications with the potential to replace incandescent and fluorescent lamps.
As is well known to those having skill in the art, a light emitting diode generally includes an active region fabricated from a material having a suitable bandgap such that electron-hole recombination results in the generation of light when current is passed through the device. In particular, materials in the Group III-nitride material system, such as GaN, InGaN, AlGaN, InAlGaN, etc., have been proven useful for generating blue, green and ultraviolet light with relatively high efficiency.
Group III-nitride based LEDs may be fabricated on growth substrates (such as a silicon carbide substrates) to provide horizontal devices (with both electrical contacts on a same side of the LED) or vertical devices (with electrical contacts on opposite sides of the LED). Moreover, the growth substrate may be maintained on the LED after fabrication or removed (e.g., by etching, grinding, polishing, etc.). The growth substrate may be removed, for example, to reduce a thickness of the resulting LED and/or to reduce a forward voltage through a vertical LED. A horizontal device (with or without the growth substrate), for example, may be flip chip bonded (e.g., using solder) to a carrier substrate or printed circuit board, or wire bonded. A vertical device (without or without the growth substrate) may have a first terminal solder bonded to a carrier substrate or printed circuit board and a second terminal wire bonded to the carrier substrate or printed circuit board.
Attempts to improve the light output of Group III nitride based devices have included providing differing configurations of the active regions of the devices. Such attempts have, for example, included the use of single and/or double heterostructure active regions. Similarly, quantum well devices with one or more Group III nitride quantum wells have also been fabricated. While such attempts have improved the efficiency of Group III nitride based devices, further improvements may still be achieved.
In particular, one problem that has been experienced with Group III-nitride devices is that of current droop, which refers to a phenomenon in which light output increases with current density up to a point, and then begins to level off. Thus, device efficiency may drop off at higher currents. Although not bound by any particular theory, it is presently believed that current droop may be the result of one or several factors, including saturation of hole injection and/or inefficient (i.e., non-light generating) electron-hole recombination at higher device currents.